The present invention relates to a method for heat-strengthening glass sheets, wherein a glass sheet is heated close to a softening temperature and then cooled at a certain controlled cooling rate. The invention relates also to an apparatus for heat-strengthening glass sheets, said apparatus comprising
a furnace which is provided with heating elements for heating glass sheets close to a softening temperature, PA1 a cooling station which is provided with nozzles above and below a glass sheet for blowing a cooling gas, PA1 a blower which is connected to said nozzles by way of a manifold, and a conveyor for carrying glass sheets in horizontal direction through the furnace and the cooling station.
The heat-strengthening of glass differs from tempering in the sense that the cooling occurs at a substantially slower rate, which also substantially reduces the surface tensions of glass. The characteristics of heat-strengthened glass are described e.g. in the published GB application 2 191 998. In order to produce standardized heat-strengthened glass, it is necessary that the cooling rate be accurately controlled. A particular problem here is that, as the thickness of glass changes, the cooling rate also changes substantially (if cooling conditions remain constant).
On the basis of earlier experiences, it seems that e.g. 8 mm glass is heat-strengthened to the Japanese standards by bringing it to the room temperature after heating. This situation appears from the set of heat-strengthening curves shown in the accompanying FIG. 6. The figure shows that, if 10 mm glass is treated the same way (cooling at a room temperature), it is already subjected to an excessive heat strengthening. Thus, if 10 mm or 12 mm glass is to be heat-strengthened, this must be carried out in an environment hotter than a room temperature. The following estimates deal with the temperatures of a cooling environment required in heat strengthening. If glass is in slow movement in a quiet environment at a room temperature, according to performed measurements, it cools with a heat-transfer coeeficient of 45 W/m.sup.2 K. Thus, the glass delivers heat at a rate of 50 kW/m.sup.2. Supposing that the delivered heat capacity of 10 mm glass is obtained approximately as inversely proportional from glass thicknesses: ##EQU1## Thus, the temperature difference between glass and ambient air will be ##EQU2## In a corresponding calculation on 12 mm glass, T.sub.air will be 250.degree. C. FIGS. 7 and 8 illustrate by way of an example the heating and heat-strengthening curves for 10 mm and 12 mm glasses. The curves reveal that e.g. 12 mm glass must remain within a 250.degree. C. temperature environment for about 250 seconds. On the other hand, a maximum loading delivers heat at a rate of appr. 400 kW. In other words, if heat strengthening is effected in a closed environment, heat must be removed from the environment at a rate of appr. 400 kW in order to maintain the environment at a constant temperature. This corresponds to the supply of 20.degree. C. air into the environment at a rate of 2 m.sup.3 /s, since at a rate of 400 kW air can be heated at 2 m.sup.3 /s from 20.degree. C. to 250.degree. C. Accordingly, in the case of 10 mm glass, the maximum loading delivers heat at a rate of appr. 450 kW, which corresponds to the raising of the temperature of appr. 3.7 m.sup.3 /s air flow from 20.degree. C. to 150.degree. C.
As pointed out above, the heat strengthening of e.g. 12 mm glass must be carried out at about 250.degree. C. in an environment with no major air movements. If the heat strengthening is carried out in a furnace, the only possible approach is probably to supply into the furnace a sufficient amount of air at a room temperature in order to maintain temperature of the furnace air at equilibrium. On the other hand, this causes a strong movement of air, whereby the coefficient of heat transfer .alpha. is considerably increased, approximately doubled. Thus, the cooling rate will be too rapid and the actually required air temperature is in the order of 500.degree. C. In fact, a problem here is to control the air input in a manner that .alpha. will remain approximately the same all over the glass and that the air flow coming into contact with glass has reached the same temperature everywhere in itself. Another problem is an increased transfer of heat caused by massive ceramic rolls, the problem being how to obtain the same cooling rate on the top and bottom surfaces of glass. A third problem is a possibility of glass breaking. It is an estimate that, on the average, every fiftieth glass is broken. This means that a furnace should be fitted with a scrap conveyor and, furthermore, a furnace must be quickly openable, so that the harmful shattered bits and pieces can be removed from between the rollers or from top of the lower resistances. A fourth problem is the inflexibility of a furnace to varying production: if heat-strengthened glass is produced today, today is no good for any other production since raising the furnace temperature back to the 700.degree. C. temperature takes a long time. Furthermore, if one of the chambers of a dual-chamber furnace is used for heat strengthening, it means the heating of glass can only be effected in the other chamber and the lower temperature of the first chamber cannot be exploited. A result of this is then that the heating of largesize glasses becomes essentially more difficult.
An object of the invention is to provide a method and an apparatus for heat-strengthening glass sheets without the above problems. A particular object of the invention is to provide a method and an apparatus, whereby the cooling of even rather thick (.gtoreq.10 mm) can be carried out in a controlled fashion, i.e. at a sufficiently slow and uniform rate over the entire surface area of a glass sheet.
Another object of the invention is to provide a method and an apparatus capable of producing also tempered glass in addition to heat strengthening.
A particular additional object of the invention is to provide a method and an apparatus, capable of flexible production, i.e. also in small series, of both heat-strengthened and tempered glass with a varying glass thickness.
These objects are achieved in the invention on the basis of the characterizing features set forth in the annexed claims.